With regard to images of an onboard camera and a security camera capable of photographing at nighttime, technology to show up an object and display it in color even at nighttime, by projecting visible light and infrared light, and picking up images by a visible sensor having a sensitivity in a visible region and an infrared sensor having a sensitivity in an infrared region, is known. Patent Document 1, for example, discloses an image signal processing apparatus in which a parameter, allowing an infrared light component contained in a signal passing through a visible light transmissive long pass filter to become almost zero, is calculated by applying transmittance data in an infrared light region of the visible light transmissive long pass filter and of an infrared light transmissive long pass filter, and the infrared light component contained in the signal passing through the visible light transmissive long pass filter is removed by applying the calculated parameter, so as to generate a visible light (RGB) image with high quality.
Meanwhile, Patent Document 2 discloses an image input device having an imaging element, in which at least three kinds of pixels having different spectral sensitivities are arranged, for picking up an image of original image data containing at least three types of original image components, and a color space converting unit for converting the original image data into color space containing a luminance signal and a chromaticity signal, so as to obtain a clear color image even at nighttime by using a visible light component and an infrared light component obtained from the imaging element.
Moreover, the so-called back view camera is known, which can photograph, the rear of a vehicle in order to support driving of the vehicle in reverse. An image photographed by the back view camera is displayed on a display device near a driver's seat, and visually confirmed by a driver and the like, so as to ensure safety. Under a relatively high luminance environment such as during daytime, obstacles including humans and animals can be visually confirmed as visible light images with ease. However, under a low luminance environment such as during nighttime, there is a problem that the obstacles including the humans and the animals are hardly confirmed visually due to the low luminance of the visible light images. It should be noted that similar problems are caused in the security cameras and the like.
In view of the problem like this, such technology has also been developed as to display an image with improved visibility even under the low luminance environment on the display device by irradiating the rear of an automobile with infrared, light by an infrared LED mounted on the vehicle, photographing the rear of the vehicle by using a near-infrared camera, and performing image processing of the photographed image. However, the infrared light is strongly reflected from a concrete wall and the surface of an asphalt pavement in the infrared light image displayed on the display device. Therefore, when the obstacle such as the human and the animal is photographed with such a background, the infrared light image overlaps the infrared light image of the wall or the surface, which causes flared highlights and results in a problem that visual confirmation of the obstacle becomes difficult.
In response to this, Patent Document 3 discloses a photographing device configured to be able to switch between an infrared photographing mode for photographing while performing infrared light irradiation and a normal photographing mode for photographing without performing infrared light irradiation. The photographing device is configured to set the luminance of a criterion color as criterion color luminance and the luminance of a reference color as reference color luminance, and to switch the infrared photographing mode to the normal photographing mode when a ratio of the criterion color luminance to the reference color luminance exceeds a predetermined range.
Meanwhile, Patent Document 4 discloses such technology that a rear view display device picks up a visible light image and a near infrared light image separately, and displays the visible light image except when the image is automatically switched to the image by the near infrared light at the time when it is sensed that headlights are turned on by ON/OF information of the headlights.